In a warning shot to wavering Democrats, the progressive action group MoveOn.org is making a major push to raise money on behalf of primary challengers to those House lawmakers who vote against health care reform.

The group is set to blast out an email to its five million member list Monday asking recipients to pledge anywhere from $25 to $200 (or more) for the purposes of defeating conservative Democrats who help defeat the legislation.

"Health care reform is in serious danger in the House of Representatives: with a handful of conservative Democrats wavering, we don't yet have the votes to pass the final bill," reads the email, which was sent in advance to the Huffington Post. "So we're asking every MoveOn member: will you pledge to support progressive primary challengers to House Democrats who side with Republicans to kill health care reform?

"With the big vote happening as early as this Friday, conservative Democrats need to know the stakes if they choose to side with Big Insurance over the voters on health care reform," the email goes on. "Our pledge will send that message loud and clear. We'll publicize the amount pledged, and make sure the media and every wavering representative know about it."

The fundraising drive by MoveOn is one of the more direct and public efforts at political intimidation yet in what is now the crucial last stage of the health care debate. Similarly, affiliates of the SEIU have been telling fence-sitting lawmakers that they will either sit out their elections or actively campaign against them should they oppose the bill.

The campaign also is a reflection of the extent to which the formal progressive movement has rallied around the legislation -- despite the deep disappointments many still have with the actual policy. Eighty-three percent of MoveOn members support the bill, even without the public option.

This is how it's done. Move On acts at the behest of its membership, which registers in the millions, and they want the bill passed. Since it's only Democrats who are in play, threatening primary challenges is one of the legitimate, institutional ways to do it.